+1 for jaywalker's comments. I have used a down bag for about forty years, both backpacking and bike touring and I soaked one exactly once - valuable lesson! A thin plastic bag liner inside your sleeping bag's stuff stack will keep things in good shape. Do not depend upon your pack or panniers to much water out. Well made down bags are durable. I used one for sixteen years, and I just this year supplemented my Marmot, twenty-three years old, with a new generation bag. The Marmot bag was hideously expensive to purchase, but became a bargain, considering all the comfy nights it gave me.

The shape and color of your tent is a lot more important than your sleeping bag color in keeping out of sight. Even more important is your choice of campsite. I agree heartily with you about staying hidden when out on a bike